Various charge control agents have hitherto been used for controlling a quantity of electrification of a toner. Known negative charge electrification controlling agents include colored compounds, such as chromium complex salts of azo dyes and chromium complex salts of aromatic hydroxycarboxylic acids, and colorless compounds, such as aluminum, zinc or boron complex salts of aromatic hydroxycarboxylic acids or aromatic dicarboxylic acids.
Although the colored charge electrification controlling agents possess appreciable effects to impart negative chargeability to toner particles, they could not be used for a color toner, particularly full color toners of three primary colors for providing a full color image or, if find any use, only provide an image of very poor color reproduction.
On the other hand, the colorless charge control agents do not have sufficient effects to impart negative chargeability to toner particles and, in particular, the resulting toners lack stability of chargeability against environmental changes especially when repeatedly used for a long period of time.
Moreover, since many metals inclusive of chromium and zinc are candidates for sources of heavy metal pollution, there is a social demand to avoid use of these metals from the standpoint of environmental conservation.